FIG. 1 illustrates a typical manufacturing apparatus of a porous glass base material. In such a manufacturing apparatus, a base member glass rod 1 is axis-rotatably disposed, and a burner 2 is provided in a lower section. The burner 2 repeatedly moves back and forth reciprocates along the longitudinal direction of the base member glass rod 1, and ejects glass particles produced by flame hydrolysis onto the lateral surface of the base member glass rod 1. The glass particles are deposited sequentially in the longitudinal direction, thereby forming a porous glass soot 3.
Above the porous glass soot 3 are provided an exhaust hood 4 and an exhaust pipe 5 so as to oppose the burner 2. The exhaust hood 4 and exhaust pipe 5 are used to emit a combustion waste gas and unattached glass particles outside. Here, if the exhaust gas flow is disturbed within the apparatus some problems may occur. For example, the soot may break while the deposition is being performed, or a bubble may be created as a result of vitrification into a transparent glass because non-deposited glass particles are later deposited.
To deal with these problems, Patent Document 1 proposes the following method. The porous glass soot 3 is housed within a tubular member 6, and the exhaust pipe 5 is provided in substantially the middle of the tubular member 6. This enables the exhaust gas to flow in a direction along the longitudinal direction of the porous glass soot 3, thereby reducing bubbles. Here, the reference numeral 7 indicates a gas inlet (see FIG. 2).
[Patent Document 1] Unexamined Patent Application Publication No. S59-190232